Stocks traded modestly off their session highs and mixed at midafternoon Tuesday. The S&P 500 added 0.3%, while the Dow Jones industrial average tacked on 0.2%. The Nasdaq dipped 0.1%. Volume was tracking higher from Monday's levels. U.S. Silica (SLCA) lost 3% in big volume, but it has staged an upside reversal after falling as much as 15% intraday. After dropping below its 50-day moving average earlier, the February IPO is now back above that key